NGC891 & M33
An interesting experiment is to set your camera to manual aperture and see what difference 1 aperture stop makes - not as lot! After all going from 8" to 12" is only 1 stop, going from 10" to 12" is les than half a stop -you can hardly see that with a camera. I used to have a rule of thumb that if you up-aperture you need to go 2 stops, hence years ago I went from 4" to 8". It is more about contrast, and as you say, in our skies, the sky-glow limits that. The same thing applies to image intensifiers - they don't increase contrast, which is what you want.
I know! I know! I recently bought a 12" newt which is only 1 stop up on my 8" SCT - but that was more along the lines of not being able to resist a bargain, and wanting a simple dob!
You do gain a bit with aperture as you can increase magnification for the same contrast but it isn't much.
You CAN increase contrast to a certain extent by taking pics and doing a bit of image processing with GIMP say. For example take a look at the pictures of NGC891 and M33 below that I recently took with my PD camera. These were only a few 20-sec exposure frames stacked - but then processed with GIMP. I have always had difficulty even seeing these with the old eyeball!